The invention generally relates to a technique and apparatus to perform a leak off test in a well.
A typical system for drilling an oil or gas well includes a tubular drill pipe, called a “drill string,” and a drill bit that is located at the lower end of the string. During drilling, the drill bit is rotated to remove formation rock, and a drilling fluid called “mud” is circulated through the drill string for such purposes as removing thermal energy from the drill bit and removing debris that is generated by the drilling. A surface pumping system typically generates the circulating mud flow by delivering the mud to the central passageway of the drill string and receiving mud from the annulus of the well. More specifically, the circulating mud flow typically travels downhole through the central passageway of the drill string, exits the drill string at nozzles that are located near the drill bit and returns to the surface pumping system via the annulus. A downhole mud pulse telemetry tool of the drill string may modulate the circulating mud flow for purposes of communicating information to the surface relating to sensed downhole formation properties, the orientation of the drill string, etc.
One technique to rotate the drill bit involves applying a rotational force to the drill string at the surface of the well to rotate the drill bit at the bottom of the string. Another conventional technique to rotate the drill bit takes advantage of the mud flow through the drill string by using the flow to drive a downhole mud motor, which is located near the drill bit. The mud motor responds to the mud flow to produce a rotational force that turns the drill bit.
The drilling of the wellbore may be interlaced with operations to install segments of a casing string, which lines and supports the wellbore. More specifically, the drilling and casing installation operations may involve the following repetitive sequence: a particular segment of the wellbore is drilled; a casing section is next run and cemented in the newly drilled segment of the wellbore, and thereafter, the drilling of the next wellbore segment may begin.
During drilling, care typically is exercised to prevent the downhole pressure that is exerted by the drilling mud from exceeding a fracture initiation pressure of the formation. More specifically, if the downhole pressure that is exerted by the drilling mud exceeds the fracture initiation pressure, the formation that is exposed to this pressure begins to physically break down and allow mud to flow into the fractured formation. Such a condition may result in damage to the formation as well as create a hazardous drilling environment. Therefore, after the casing shoe (the lower bullnose end) of the most recently installed casing string segment is drilled out by the drill bit a test called a formation integrity test, or “leak off test” (LOT), typically is performed for purposes of determining the fracture initiation pressure for the next segment of the wellbore to be drilled. The LOT also provides a way to test the integrity of the cementing on the most recently installed casing section.
A typical LOT involves sealing off the annulus of the well and introducing drilling mud at a relatively slow and constant volumetric rate through the central passageway of the drilling string so that the mud exits the string near the string's bottom end and enters the bottom hole region of the well. During the LOT, the introduction of the mud flow gradually increases the bottom hole pressure due to the sealed annulus. The pumping of the drilling mud continues until either a predetermined test pressure is reached or the loss of drilling fluid into the formation is detected. The pressures and flow rates associated with the LOT typically are measured using sensors that are located at the surface of the well.